The Scientific Group on Methodologies and Safety Evaluation of Chemicals (SGOMSEC) held its 3rd Workshop in August 1983 in Guildford, U.K., to develop test methodologies for chemical mixtures. For the section of the Workshop on Evaluation of Mixtures, a manuscript was prepared on predicting concentrations of air pollutants. The manuscript reviewed the meteorology and atmospheric chemistry of photochemical smog and the mathematical models used to predict ground level concentrations of pollutants. From this review, it was recommended that techniques be developed to measure the variability of velocity fields and dispersion coefficients for long-range transport of atmospheric pollutants and the measurement of transient free radicals and other very reactive chemicals that participate in the atmospheric reaction of photochemical smog to measure the variability. In addition, it was recommended that methods be developed to determine when simple models and single chemical models can be used as surrogates for predicting concentration profiles for chemical mixtures.